As described in the aforesaid U.S. application Ser. No. 11/110,263 by Matthew A. Taylor, Kevin S. Bassett, and Paul E. Gili, entitled “Method and Apparatus For Transmission Line and Waveguide Testing”, and Ser. No. 11/035,311 by Joshua Niedzwiecki, entitled: “Reduced Complexity Transmission Line And Waveguide Fault Tester”, both assigned to the assignee hereof and incorporated herein by reference, multi-port junctions are used in combination with a module for generating a complex reflection coefficient from the outputs of the multi-port junction in order to establish in one embodiment the existence of faults in a transmission line and the severity thereof. In these applications, a reflectometer having a frequency source that is stepped from one frequency band to the next provides a complex reflection coefficient profile with frequency for each of the faults or discontinuities in the transmission line.
When the estimated complex reflection coefficient is processed by an Inverse Fourier Transform, then the frequency domain information is converted to time domain information. The time domain information is then converted to distance to a fault or range to a fault in the transmission line.
Algorithms described in the Taylor patent application and in the Niedzwiecki patent application include various algorithms for estimating complex reflection coefficients from respectively a six-port junction and a four-port junction in which the output signals from the multi-port junctions are functions of both the signal source and the returned reflected signal.
It will be appreciated that when multi-port junctions are utilized for reflectometers, gone are the usual IF stages, heterodyning, mixers and oscillators which are in general used to convert received signals to base band where the processing occurs.
The same heterodyning techniques are used in current stepped frequency radars in which received signals are down-converted by an intermediate IF stage to an IF frequency. The result of the down-conversion is then digitally sampled to bring it down to base band where one is able to examine the differences between what is transmitted and what is reflected.
The problem with such stepped frequency radars is the cost of such IF stages, which include expensive oscillators and mixers.
By way of further background, frequency-swept radars have been used as ground penetrating radars in which sub-surface objects are to be identified, such as land mines, pipes, voids in concrete and other subterranean objects. There are a wide variety of time domain reflectometers and systems that develop their information by ascertaining the range to the discontinuity by detecting, for instance, round trip travel times.
Where there are a number of ultra wideband ground penetrating radars that use swept frequencies, their resolution and the ability to identify subsurface objects leaves something to be desired. Others have suggested using pulsed radars as ground penetrating radars. The problem with such ground penetrating radars or through-the-wall radars that use pulse techniques is that, as one gets higher in frequency, one cannot adequately control the leading and trailing edges of the pulses so that high fidelity resolution is not possible.
It would be desirable to have a high fidelity frequency resolution in which a high fidelity map of the reflection coefficient of the returned energy across frequency is determined. However, if one were to attempt to use time domain reflectometry techniques for a ground penetrating radar, one can only achieve limited fidelity to, for instance, fully characterize the frequency response of the reflecting objects.
It is therefore impossible utilizing traditional time domain reflectometers to obtain a high fidelity radar map of the reflection coefficients across frequency.
While stepped frequency radars have been used in the past that are frequency agile, it is important to be able to ascertain with high fidelity what is happening in the main lobe of the radar using sensitive techniques that require phase coherence of the radar itself. How one accomplishes frequency stepping while maintaining phase coherence is indeed a problem and one that heretofore has required expensive equipment to be able to generate phase-coherent transmitted radiation and to be able to analyze the returns based upon samples of the transmitted radiation.
Moreover, in the past and as described in U.S. patent application Ser. No. PCT/US04/20116 by Paul Zemany entitled Dual Frequency Through-the-wall Motion Detection and Ranging Using Difference-Based Estimation Technique, filed Sep. 14, 2004, assigned to the assignee hereof and incorporated herein by reference, two-color radars have been used to be able to detect motion of individuals behind a wall in a so-called through-the-wall system. In this system, CW signals of two alternating different frequencies are used to detect the presence of moving individuals behind a wall. Moreover, when multiple frequency bands are available, one is able to locate not only the fact of a moving individual, but also the location of the moving individual. Moreover, when more and more radars surround, for instance, a building, one can triangulate to more accurately estimate the position of the individual. One therefore needs an inexpensive frequency-stepped CW radar for these applications.
Such through-the-wall systems are extremely useful for fire and police for commercial applications as well as for the military to be able to detect enemy combatants or soldiers behind a wall or within a building.
As will be appreciated, several through-the-wall applications require frequency-stepped radars and for this reason one would like to develop a relatively inexpensive, simplified frequency-stepped radar for these purposes.